Which model G5 is it? Here's the troubleshooting steps for the June 2004 and Early 2005 models.
Power-on LED illuminates when power button is pressed, fans spin, and boot tone
chimes, but there is no video
Reseat video card. (Make sure video card is fully inserted in connector and end of card is
secured by the connector latch.)
Reset PRAM (restart computer while holding down Command-Option-P-R keys until second
boot tone chimes)
Replace video card
Reset logic board. Refer to “Resetting the PMU on the Logic Board” in this chapter.
Replace logic board
Resetting the PMU on the Logic Board
The PMU (Power Management Unit) is a microcontroller chip that controls all power functions for
this computer. The PMU is a computer within a computer. Its function is to:
tell the computer to turn on, turn off, sleep, wake, idle, etc.
manage system resets from various commands.
maintain parameter RAM (PRAM).
manage the real-time clock.
Important: Be very careful when handling the logic board. The PMU is very sensitive and
touching the circuitry on the logic board can cause the PMU to crash. If the PMU crashes and is
not reset, the battery life goes from about five years to about two days.
Note: For the location of the PMU reset button, see “Logic Board Diagram” in the Views chapter.
Many system problems can be resolved by resetting the PMU chip. When you have a computer
that fails to power up, follow this procedure before replacing any modules:
1. Disconnect the power cord and check the battery in the battery holder. The battery should
read 3.3 to 3.7 volts. If the battery is bad, replace it, wait ten seconds, and then proceed to
step
2. If the battery is good, go directly to step 2.
Press the PMU reset button once and then proceed to step
3. Do not press the PMU reset
button a second time because it could crash the PMU chip.
Wait ten seconds before connecting the power cord and powering on the computer. If the
computer does not power on, there is something else wrong with it; refer to the “Startup
Failures” section of “Symptom Charts” in this chapter.